test: add isolated real-tmux smoke tests

Drive TmuxRunner against a throwaway tmux server (isolated via TMUX_TMPDIR)
so the tests never touch the user's live tmux. Register the 'tmux' pytest marker.
Skips when no tmux binary is present.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pyproject.toml

@@ -27,6 +27,11 @@ dev = [
     "ty>=0.0.47",
 ]
 
+[tool.pytest.ini_options]
+markers = [
+    "tmux: real-tmux smoke tests; require a tmux binary and run on an isolated TMUX_TMPDIR server",
+]
+
 [tool.hqt.quality]
 required_after_code_or_test_changes = [
     "uv run ruff format src tests",